The present invention relates to a method for extracting power from attached signals to provide operating power for small intelligent (that is, incorporating a microprocessor) computer accessory devices and to an accessory device incorporating the method. Such devices provide, for example, interconnection between computers and between computers and peripheral devices. Typical peripheral devices include computer data protocol convertors, printer data buffers, modems and printer and computer sharers. Such accessory devices, due to the moderate power levels required to operate the internal electronics are typically powered by an AC mains plug pack or separately packaged AC mains transformer.
The expansion of the use of portable battery powered note book computers and portable battery powered printers to provide complete independence from the AC mains has of course meant that these intelligent computer accessory devices cannot be used with a note book computer and battery printer without close proximity to an AC mains supply to power the accessory.
These separate AC mains transformers or plug packs are also rather cumbersome and are clumsy to install when used in transient locations. Thus the disadvantages are that they are required at all when all other devices are battery powered and that they are inconvenient to use.
A very typical characteristic of these computer accessory devices is that they are sold into a very price sensitive market and so the cost of providing a power source must be absolutely minimized.
Firstly it should be emphasized that the accessory device to which the present invention applies is one that employs a microprocessor to achieve the required functionality. Such a microprocessor could easily draw 100-200 mW. A class of "dumb" signal data powered computer interconnection accessory that requires very little power has existed for some time. These are short haul line drivers that extend the distance over which RS232 signals may be transmitted.
As mentioned above the typical power source for known computer accessories is an external AC mains transformer or plug pack. No extra explanation is felt necessary as this class of power source is well established. Some computer accessories include a provision for replaceable or rechargeable batteries within the body of the accessory. This method is not widely used. The user of such a computer accessory is forced to keep account of yet another set of batteries which may explain why this method has lacked popularity.
Some particular brands of computers and peripherals such as printers provide an auxiliary current limited power source on unused pins within the interconnection sockets. Such a source of power could easily be adapted to power interconnection accessories. The problem with use of such a power source is that computers and peripherals that provide such auxiliary power are not common and the pins of the inter-connection socket used for this power are not standard. Any product developed using such a power source would only have a limited and specific application.
The problem of providing power does not lend itself to a traditional solution such as rectifying the incoming serial data signal and feeding this to a switch mode transformer coupled DC/DC convertor, as such a switch mode power supply will not start up with the relatively high source resistance of the serial data input. Also such a switch mode power supply will be inefficient at these voltages and power levels, expensive, and will be physically large.